Over the past several decades, the telecommunications industry has witnessed the extremely rapid growth and diversification of interconnect services for a continuously expanding base of users of digital terminal equipment, such as personal computers. When initially faced with the problem of providing communication connectivity between such digital terminal equipment and the already in-place analog POTS (plain old telephone service) network, the industry began offering digital/analog interfaces, or modems, to network customers Current modem designs allow a digital terminal equipment user to choose from a variety of types and speeds, including modems that may be installed internally with the data terminal equipment, or those that are operated as external, stand-alone devices
Now, although a modem allows the digital terminal equipment user access to an analog POTS network, the signal quality and data transport speed of analog circuits are limited, making such circuits less than ideal media for digital data communications. Indeed, faced with the vigorous demand for digital communication services by effectively every business, professional, educational and social institution, communication service providers now offer dedicated digital circuits (such as integrated services digital network (ISDN) links). Since digital circuits are capable of transporting multiple time division multiplexed digital channels at very high data rates, they have become the framework for what is currently known as the "internet".
Because ISDN communications are purely digital and employ a predefined data assembly and encoding format, an ISDN line cannot be directly accessed by either the serial digital port or the analog port of a conventional modem Instead, it is necessary for a user to install an ISDN terminal adapter between the user's digital terminal equipment and the ISDN circuit/line Compounding the customer's problem is the fact that many commercial information service providers (e.g., America On Line and Compuserve) use analog circuits, which are accessed by the use of a conventional modem Thus, for the foreseeable future it is expected that digital terminal equipment users will require access to both analog and digital communication networks. One proposal to meet this two-fold need is to provide an `integrated` ISDN terminal adapter, which is selectively operative under software control to provide either conventional modem-based connectivity or all digital (ISDN-based) connectivity. For this purpose, as diagrammatically illustrated in FIG. 1, such an integrated ISDN terminal adapter 10 provides two data transmission paths therethrough between a user's data terminal equipment (DTE) 14 and a local subscriber loop 16. The first transmission path comprises a modem-based connectivity path 11, which is employed to provide conventional modem connectivity between the subscriber loop and the customer's data terminal equipment. The second transmission path comprises an all digital signalling path 12, which is employed to provide digital-digital connectivity between an ISDN link and the user's data terminal equipment.
For this purpose, the terminal adapter 10 includes a digital communication transceiver, for example an ISDN transceiver 30, which is externally ported (on the network side of the adapter) to the subscriber loop 16 via a connector 22, such as an RJ-45 connector, and is internally ported to a time division multiplex (TDM) digital communication bus 31. The ISDN transceiver 30 is operative to transmit digital (ISDN-formatted) communication signals onto the local loop 16 and is operative to receive ISDN-formatted communication signals from the loop. To provide modem-based connectivity through the first path 11, the TDM communication bus 31 is coupled via a coder-decoder (CODEC) 33 and an associated modulator-demodulator (modem) chipset 35 to a data bus 37 of an internal serial communication exchange processor 40. Also coupled to data bus 37 are an EPROM 36 and RAM 38 associated with the execution of the communication software that drives the operation of communication processor 40.
The serial communication exchange processor 40 is equipped with a plurality of (three) serial communication controllers (SCCs) 41, 42 and 43, which are operative, under internal processor control, to provide serial digital communication connectivity between digital signal transport devices to which the SCCs are ported. In particular, SCC 41, used for terminal data connectivity with the user's data terminal equipment 14, is coupled via a serial data (such as RS 232 digital data) communication equipment (DCE) link 51 to a digital communication connector 21, such as a DB 25 connector, to which the user's data terminal equipment 14 is connected SCC 42, used for bearer (B) channel data, and SCC 43, used for signalling (delta (D) channel) data, are coupled to the TDM bus 31. SCCs 41, 42 and 43 provide the digital-digital connectivity of the second path between the user's data terminal equipment 14 and the ISDN transceiver 30.
The terminal adapter 10 is also equipped with a third transmission path therethrough which provides analog-based signalling connectivity between subscriber loop connection port 22 an analog signal connection port 23, such as a standard RJ-11 telephone jack, to which customer analog communication equipment, shown at 15 and 17, is connected. The third transmission path includes a bidirectional analog-to-digital, digital-to-analog conversion unit (DAC) 60, such one including as a two-to-four wire hybrid 61 that is coupled to analog signalling connector 23, and an associated coder/decoder (CODEC) 63, which is coupled, to the TDM bus 31, so that it may interface digitized (bearer) channel data between the transceiver 30 and the DAC 60.
The communication control software which-directs and supervises the operation of the terminal adapter of FIG. 1 is configured to provide communication connectivity between the user's data terminal equipment 14 and the ISDN line 16, based upon the format of the data signals that are sourced from or intended for the user's equipment. Selection of the appropriate path (modem-based path 11 or ISDN-based path 12) through the integrated terminal adapter is carried out by the terminal adapter's control processor, rather than the customer, who is usually technically unsophisticated and accustomed to doing nothing more than simply installing an analog modem in the customer's premises-located equipment, and plugging in a telephone connector to a modem port.
For example, in the case of a subscriber to an all digital communication service (an internet provider) placing a digital-digital call (such as an ISDN call to an ISDN circuit employed by the internet provider), the micro-controller within the serial communication exchange processor 40 uses the SCC 43 to conduct (D) channel signalling over the TDM bus 31 to ISDN transceiver 30 to set up or establish call connectivity. Then, once call connectivity has been established, the processor provides a serial communication path between SCC 41, which is ported to the data terminal equipment 14 via connector 21, and SCC 42, which is ported to TDM bus 31 and thereby to the ISDN transceiver 30, so that serial data communication may take place over the second transmission path 12 between the data terminal equipment 14 and the subscriber loop 16 via serially connected SCCs 41 and 42.
On the other hand, when the user places a call to an analog communication service provider, (for example, the user dials a commercial internet-interface provider, such as Compuserve or America On Line), the micro-controller within the serial communication exchange processor 40 will provide modem-based connectivity between the SCC 41 and the processor's data bus 37, to which the modem chipset 35 and its associated coded 33 are coupled, so that communications will be properly formatted for transmission over the first path 11 to the analog modem-based communications provider.
Now, although such an integrated terminal adapter enables the user to access both a conventional POTS line and an ISDN line, it does so at a price, including not only the fact that the customer's modem, which had previously given the user at least POTS line connectivity, is now treated as nothing more than an unused relic, but the user must now bear the added cost of the internal modem chipset.